The present invention relates to a method of preparing particularly skin-compatible cosmetic or dermatological cleansing preparations.
Preparations of this type are, for example, foam baths and shower preparations, solid and liquid soaps or co-called xe2x80x9csyndetsxe2x80x9d (synthetic detergents), shampoos, handwash pastes, personal hygiene washing compositions, special cleansers for small children and the like.
In a particular embodiment, the present invention relates to cleansing preparations for use as shower preparations, bath preparations, body cleansing and face cleansing.
Preparations of this type are also known per se. They are essentially surface-active substances or substance mixtures supplied to the consumer in a variety of preparations. Preparations of such type are generally distinguished by a greater or lesser water content, but can also, for example, be in the form of concentrate.
Even simple bathing in water without the addition of surfactants will initially cause the horny layer of the skin to swell, the degree of this swelling depending, for example, on the bathing time and temperature. As well as water-soluble substances, e.g. water-soluble constituents of dirt, substances endogenous to the skin which are responsible for the water-binding capacity of the horny layer are also washed off or out. In addition, as a result of surface-active substances endogenous to the skin, fats in the skin are also dissolved and washed out to a certain degree. After the initial swelling, this causes a subsequent significant drying-out of the skin, which may be further intensified by washing-active additives.
In healthy skin, these processes are generally of no consequence since the protective mechanisms of the skin can readily compensate for such slight disturbances to the upper layers of the skin. However, even in the case of nonpathological deviations from the norm, e.g. as a result of wear damage or irritation caused by the environment, photodamage, aging skin etc., the protective mechanism of the surface of the skin is impaired. In some circumstances it is then no longer able to fulfill its role by itself and must be regenerated by external measures.
The object of the present invention was therefore to remedy these shortcomings of the prior art. It was further an object of the invention to provide bath or shower preparations which on the one hand have a high care action, without, on the other hand, the cleansing action becoming inferior.
Surface-active substances, the best known being the alkali metal salts of higher fatty acids, i.e. the classical xe2x80x9csoapsxe2x80x9dxe2x80x94are amphiphilic substances which are able to emulsify or solubilize organic nonpolar substances in water.
These substances not only flush dirt from the skin and hair, they irritate skin and mucous membranes to a greater or lesser extent depending on the choice of surfactant or surfactant mixture.
One of the most commonly used surfactants for cosmetic compositions throughout the world is sodium lauryl ether sulfate. Although an excellent washing-active agent with good foaming ability, in higher concentrations it has an irritative effect on skin and mucous membranes.
As more recent investigations show, the irritancy potential of sodium lauryl ether sulfate is at least partially promoted by the fact that this substance binds to the surface of the skin where it forms a certain reservoir. Studies suggest that lauryl ether sulfate from this reservoir penetrates into the deeper layers of the skin, where it can then cause uncontrolled secondary reactions, which harbor an increased risk of irritation.
Commercially available sodium lauryl ether sulfate (=sodium polyoxyethylene lauryl sulfate, according to INCI nomenclature: xe2x80x9cSodium laureth sulfatexe2x80x9d; CAS No.1335-72-4), like most raw materials used in cosmetics, is not a pure substance, but, depending on the preparation, is more likely a mixture of substances, the structures of which conform to the general formula 
where n assumes numbers from 0 to 10 and m assumes numbers from 4 to 6. The lauryl ether derivative which predominates in the commercial products and gives them their name has m=5, n=2-3. Commercial products are, for example, Texapon(copyright) N 25, Texapon(copyright) N 40, Texapon(copyright) N 70 and Texapon(copyright) N 103 from Henkel KGaA.
There are, however, also other lauryl ether sulfates which have, as counterion, for example, unsubstituted ammonium ions or ammonium ions substituted by alkyl groups or hydroxyalkyl groups, but also magnesium and such like.
However, because of the ready availability, the acceptable price and the excellent washing properties of sodium lauryl ether sulfate, it will not be possible in practice to dispense with this substance entirely in the foreseeable future. Although lauryl ether sulfate-free preparations are known and entirely advantageous, they are characterized by other performance- or preparation-related or economic disadvantages.
The long-term application (for example longer than 1 hour) or repeated short-term application of anionic surfactants can lead to a reduction in skin moisture or to an increase in the transepidermal water loss (TEWL).
It is known per se to use sodium lauryl ether sulfate in combination with other surfactants as washing-active agent. The person skilled in the art who would then like to increase the skin compatibility of such preparations then replaces some of the sodium lauryl ether sulfate with milder surfactants. However, a reduction in foaming and/or cleansing performance usually has to be accepted as an undesired side effect. The aim was therefore to remedy this shortcoming.
In the attempt to increase the compatibility of lauryl ether sulfate on the basis of scientific laws without replacing the surfactant with other, better tolerated surfactants (=cosurfactants), the person skilled in the art is faced with the following, as yet unresolved, contradiction:
The skin compatibility of surfactants correlates with the monomer concentration/CMC of the surfactants. It does not increase further at concentrations above the CMC.
Imokawa G, Mishima Y. Contact Dermatitis. 1979: 5: 357
Breuer M M. J Soc. Cosmet. Chem. 1979: 30: 41
The degree of skin damage increases with increasing surfactant concentrationxe2x80x94even above the CMC.
Wilhelm K P, Surber C, Maibach H L. Arch. Dermatol. Res. 1989: 281: 293-295.
Because of this contradiction, it was not clear to the developer which laws can be used to prepare a milder formulation of a given surfactant systemxe2x80x94in this case lauryl ether sulfate.
In various publications it has been speculated that there is a connection between the skin reaction and the adsorption of surfactants on the skin. However, all of the investigations relating to the adsorption of surfactants on the skin have been carried out either
in vitro on substances with limited similarity to the human skin (callous powder, skin powder, isolated human or mammal Stratum corneum)
Dominguez J G, Parra J L, Infante M R, Pelejero C M, Balaguer F, Sastre T. J Soc. Cosmet. Chem. 1977: 28:165
Garret H E. Trans. of St. John""s Hosp. Dermatol. Soc. 1965: 51: 166
Faucher J A, Goddard E D. J Soc. Cosmet. Chem. 1978: 29: 323
Gibson W T, Teall M R. Fd Chem. Toxic. 1982: 21: 581
ex vivo (excised human or mammal skin)
Fullerton A, Broby-Johansen U, Agner T. Contact Dermatitis. 1994: 30: 222
or
in vivo using inadequate, irrelevant, due to being application-remote, or unvalidated methods (indirect dyeing methods, extraction with water or acetone)
Imokawa G, Mishima Y. Contact Dermatitis. 1979: 5: 357.
Adsorption measurements carried out under equilibrium conditions (long-term application) are of no relevance for application conditions (showering). Measurements described in the literature do not therefore adequately reflect the application situation and therefore produced irrelevant results.
At the 2nd Scientific Conference of the Asian Societies of Cosmetic Scientists, 1995, in Seoul, a paper entitled xe2x80x9cDevelopment of High-Safety Facial Cleansers through Reduction of Cutaneous Surfactant Adsorptionxe2x80x9d was presented which dealt with a theme related to the present invention.
However, the xe2x80x9cfacial cleansersxe2x80x9d are soap products which, however, are not intended to be the subject of our patent. Although soaps are also anionic surfactants, they can only develop their surface-active action at a high pH. Upon contact with the skin, soaps can lose their basicity as a result of the buffering action of the skin and as a result lose their soap character. The synthetic detergents on which we focus do not have this peculiarity.
By virtue of this limitation, soaps exhibit unique characteristics and, when compared with synthetic detergents such as lauryl ether sulfate or SLS, are to be regarded as an independent product class.
The method which we have developed for determining the adsorption of lauryl ether sulfate on the skin simulates a washing operation as occurs, for example, during daily showering:
Firstly, the inner sides of the forearms are divided into a number of test areas using insulating tape. The area to be treated is then wetted with approximately 50 ml of mains wafer (T=38xc2x11xc2x0 C.). The lauryl ether sulfate is then applied. Throughout the application period of 45 s, the test subject distributes the lauryl ether sulfate over the area using 2 fingers in uniform circular movements. At the end of the application period, the test area is rinsed with approximately 950 ml of mains water (T=38xc2x11xc2x0 C.) and carefully patted dry using a clean cellulose cloth. For the desorption, a plastic ring with an internal diameter of 25 mm is pressed firmly onto the inner side of the test subject""s forearm. 1000 xcexcl of a 1% Triton X-100 solution are then introduced into the ring. The tip of a round Teflon-coated spatula is then used to scrape the skin uniformly for 1 min. This leads to suspension of corneocytes. To work up the samples for analysis, the samples are centrifuged and 800 xcexcl of the supernatant are drawn off. The supernatant is analyzed with regard to its lauryl ether sulfate content using ion-pair chromatography. The effectiveness of the desorption method described is 79%, i.e. 79% of the amount of lauryl ether sulfate actually present on the skin are included.
The results of the experiments for the adsorption of lauryl ether sulfate show that sodium lauryl ether sulfate adsorbs/binds to the surface of the skin during washing operations under use conditions.
Surprisingly, the amount of surfactant adsorbed on the surface of the skin under use conditions further increases even above the maximum achievable monomer concentration (CMC).
The object of the present invention was therefore to remedy the shortcomings uncovered.
Surprisingly, and herein lies the basis of the solution, the shortcomings are overcome according to the invention by a method of preparing mild washing-active cosmetic or dermatological preparations, which comprises determining the critical micelle concentration (CMC1) of one or more washing-active surfactants (surfactant A) using customary methods, subsequently choosing one or more cosurfactants (surfactant B) and, by varying the relative concentrations of surfactants and cosurfactants to one another, determining the respective concentrations and the concentration ratio to one another at which the critical micelle concentration (CMC2) assumes a desired value which is lower than CMC1, and subsequently combining the customary constituents of a washing-active cosmetic or dermatological preparation together with surfactant A and surfactant B in the concentrations determined in each case.
A further embodiment of the present invention relates to a method of reducing the binding of surfactants to the surface of the skin, which comprises determining, using customary methods, the critical micelle concentration (CMC1) of one or more washing-active surfactants which have a tendency to bind to the surface of the skin (surfactant A), subsequently choosing one or more cosurfactants (surfactant B), and, by varying the relative concentrations of surfactants and cosurfactants to one another, determining the respective concentrations and the concentration ratio to one another at which the critical micelle concentration (CMC2) assumes a desired value which is lower than CMC1, and subsequently combining the customary constituents of a washing-active cosmetic or dermatological preparation together with surfactant A and surfactant B in the concentrations determined in each case.
The invention is further embodied by the use of one or more cosurfactants (surfactant B) which do not correspondent to the surfactant sodium lauryl ether sulfate in a mixture with the surfactant sodium lauryl ether sulfate, for reducing the binding of the surfactant A to the surface of the skin.
The invention further relates to the use of one or more cosurfactants (surfactant B) for the preparation of mild washing-active cosmetic or dermatological preparations with a further content of one or more washing-active surfactants which have a tendency to bind to the surface of the skin (surfactant A), which, when used, achieve a decrease in or prevention of the binding of surfactant A to the surface of the skin.
In a particular embodiment, the present invention relates to washing-active hair cosmetic preparations, commonly referred to as shampoos. In particular, the present invention relates to hair cosmetic active ingredient combinations and preparations for the care of the scalp.
Surprisingly and unforeseeable by the person skilled in the art, it has been found that the addition of cosurfactants to lauryl ether sulfate leads to a reduction in the amount of lauryl ether sulfate adsorbed on the skin. Skin compatibility studies carried out in parallel demonstrate the increased mildness of the products as a result of the addition of cosurfactants.
Table 1 shows the reduction in the adsorption of lauryl ether sulfate as a result of the addition of different cosurfactants to a lauryl ether sulfate solution.
This result is all the more surprising since the addition of one or more cosurfactants leads to an increase in the active content of surfactants in the formulation. The person skilled in the art would expect a reduction in skin mildness as a result of this increase in concentration.
Surprisingly, the opposite result was found: the increase in the overall surfactant concentration led to an increase in the mildness of the products.
A further embodiment of the present invention relates to the use of one or more cosurfactants (surfactant B) in a mixture with one or more surfactants which differ from surfactant B (surfactant A) for decreasing the binding of surfactant A to the surface of the skin.